V4L/DVB (8389): videodev: simplify get_index()

Use Hans Verkuil's suggested method of implementing get_index which doesn't
depend on class_for_each_device and instead uses the video_device array. This
simplifies the code and reduces its memory footprint.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>